It'd be tough to overstate how much World War II still resonates in Russia. Take its naming: It's no "Second World War" in Russia, it's The Great Patriotic War. Then consider the death toll: 27 million Soviet citizens dead, according to one estimate. Horrifying events that took place on Russian soil, such as the Siege of Leningrad and the Battle of Stalingrad, are not forgotten.

This week, that's especially important. Remember that in Russia, May 9 is Victory Day: the anniversary of the day that the Soviet Union announced that Nazi Germany had surrendered. And Victory Day isn't just a big day for Russia. It's a big day for all former members of the Soviet Union who fought together in the war. This year, it may be an exceptionally complicated day for one country in particular: Ukraine.

In the past, Ukraine and Russia have celebrated Victory Day together on May 9 – for example, on May 9, 2010, Russian and Ukrainian sailors in Sevastopol marched in a joint parade to commemorate the 65th anniversary of the day on the orders of then-President Viktor Yanukovych.

But there's also been controversy. To some Ukrainians, World War II was a fight against both Nazi Germany and the Soviet Union. Ukrainian nationalists fought against the Soviet army at some points during World War II — some, such as nationalist idol Stepan Bandera, sided with Nazi Germany for part of it. Even before the Euromaidan protests spun Ukraine into crisis, there was contention over the history: In 2011, Yanukovych revoked a "Hero of Ukraine" award that had been given to Bandera by his predecessor and criticized the Western city of Lviv for failing to celebrate Victory Day appropriately.